This patch fixes couple of issues in scenario test from
test_connectivity module.
1. Replace safe_client with client object
In class NetworkConnectivityTest there was used safe_client
but there is no such attribute in this class. Object "client"
should be used instead.
2. It also fixes in the same test how external network's subnet
ID is get from the network's info.
3. Change to use admin_client to get details of external
network's subnet as this subnet don't belongs to tenant user
so regular client gets 404 error while doing subnet_show
command.
4. Check the subnets IP version to retrieve only an IPv4 one.
Change-Id: Ibebb20b29dd6ae902d194fd26ba1ea728a976286
Closes-bug: #1861670
2a71a89664